1. Field of the Invention
This invention relates to a controller of a PLL (phase locked loop) type for a servomotor, and more particularly to a controller suitable for a DC servomotor used as a drive source for a scanning unit of a copying machine with reduction and enlargement functions, or for a video/audio disc player or a video/audio tape recorder which need a plurality of rotational speeds.
2. Description of the Prior Art
An electrophotographic copying machine needs a synchronization of the circumferential speed of a photosensitive drum with a document scanning speed. Accordingly, a copying machine having reduction and enlargement functions contains a gear-system speed change apparatus interposed between a motor used as the drive source common to the photosensitive drum and the document scanning unit and the driven units (the drum and the document scanning unit) so that a gear ratio corresponding to the magnification is selected to thereby synchronize the drum and scanning unit. Recently, the choice of magnification ratios has increased, so that the gear-system speed change apparatus has become increasingly larger in size. Hence, the copying machine is provided with a servomotor used only for document scanning and driven at a predetermined speed in synchronism with the photo-sensitive drum rotating at a constant speed, thereby allowing the miniaturization of the copying machine.
Now, in order to precisely control the servomotor, a phase and speed control system is used which detects not only the rotational speed of the motor but also the phase thereof so as to control them to meet predetermined conditions. Such a system has been well-known as disclosed in the U.S. Pat. No. 3,215,918 and No. 3,621,353, the control circuits of these inventions comprising analog circuits. Recently, a digital circuit has been used for the control unit in which a servo loop employs an analog amplifier and an analog phase compensating circuit so that digital and analog devices are mixed.
Hence, since the servo loop is subjected to gain adjustment and phase compensation, a number of components, such as capacitors and resistances, are required, thereby having the defect that the number of parts increases to complicate the circuitry. Additionally, an adjusting portion is needed for adjusting the constants of the circuitry elements. Also, the control circuit is defective in that, in order to avoid such adjustment, precise computation need previously be carried out and circuitry elements of a high accuracy must be used. Furthermore, even when a similar control circuit is used, if the motor or load is different, there is difficulty in that the circuit constants must be changed corresponding to the different motor or load.